1. Field of the Invention
This invention relates to improved organic light emitting diode (OLED) display devices, and in particular to increased efficiency color display devices.
2. Related Technology
Organic light emitting diodes (OLEDs) comprise a particularly advantageous form of electro-optic display. They are bright, colorful, fast-switching, provide a wide viewing angle and are easy and cheap to fabricate on a variety of substrates. Organic LEDs may be fabricated using either polymers or small molecules in a range of colors (or in multi-colored displays), depending upon the materials used. Examples of polymer-based organic LEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of so called small molecule based devices are described in U.S. Pat. No. 4,539,507.
A basic structure 100 of a typical organic LED is shown in FIG. 1. A glass or plastic substrate 102 supports a transparent anode layer 104 comprising, for example, indium tin oxide (ITO) on which is deposited a hole transport layer 106, an electroluminescent layer 108, and a cathode 110. The electroluminescent layer 108 may comprise, for example, PPV (poly(p-phenylenevinylene)) and the hole transport layer 106, which helps match the hole energy levels of the anode layer 104 and electroluminescent layer 108, may comprise, for example, PEDOT:PSS (polystyrene-sulphonate-doped polyethylene-dioxythiophene). Cathode layer 110 typically comprises a low work function metal such as calcium and may include an additional layer, such as a layer of aluminium. Contact wires 114 and 116 to the anode the cathode respectively provide a connection to a power source 118. The same basic structure may also be employed for small molecule devices.
In the example shown in FIG. 1 light 120 is emitted through transparent anode 104 and substrate 102 and such devices are referred to as “bottom emitters”. Devices which emit through the cathode may also be constructed, for example by keeping the thickness of cathode layer 110 less than around 50-100 nm so that the cathode is substantially transparent.
Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-color pixelated display. A multi-colored display may be constructed using groups of red, green, and blue emitting pixels. In such displays the individual elements are generally addressed by activating row (or column) lines to select the pixels, and rows (or columns) of pixels are written to, to create a display. It will be appreciated that with such an arrangement it is desirable to have a memory element associated with each pixel so that the data written to a pixel is retained whilst other pixels are addressed. Generally this is achieved by a storage capacitor which stores a voltage set on a gate of a driver transistor. Such devices are referred to as active matrix displays and examples of polymer and small-molecule active matrix display drivers can be found in WO 99/42983 and EP 0,717,446A respectively.
It is usual to provide a current-controlled rather than a voltage-controlled drive to an OLED because the brightness of an OLED is determined by the current flowing through the device, this determining the number of photons it generates. In a voltage-controlled configuration the brightness can vary across the area of a display and with time, temperature, and age, making it difficult to predict how bright a pixel will appear when driven by a given voltage. In a color display the accuracy of color representations may also be affected.
FIG. 2a shows an example of a current-controlled pixel driver circuit 220 in which the current through an OLED 216 is set by using a reference current sink 220 to set a drain source current for an OLED driver transistor 212 (which in this example also flows through a switching transistor 214). The circuit memorizes the driver transistor gate voltage required for this drain-source current with a capacitor 218. Thus the brightness of the OLED 216 is determined by the current, Icol, flowing into an adjustable reference current sink 220, which is set as desired for the pixel being addressed. One current sink 220 is provided for each column data line. In the illustrated circuit all the transistors are PMOS (although NMOS transistors may also be used) and thus the source connections are towards GND and Vss is negative.
FIG. 2b shows a display driver for an active matrix display 202. In FIG. 2b the active matrix display 202 has a plurality of row electrodes 204a-e and a plurality of column electrodes 208a-e each connecting to internal respective row and column lines 206, 210 (for clarity, only two are shown). Power (Vss) and ground connections are provided to provide power to the pixels of the display. A pixel 200 is connected to the Vss, ground, row and column lines. In practice a plurality of pixels is provided generally, but not necessarily, arranged in a rectangular grid and addressed by the row and column electrodes 204, 208. The active matrix pixel 200 may comprise any conventional active matrix pixel driver circuit, such as the circuit of FIG. 2a. 
The row and column electrodes 204, 208 are driven by row and column drivers 230, 234 controlled by a display drive logic 246. As illustrated each column electrode is driven by an adjustable constant current generator 240, in turn controlled by a data output 236 from a display device logic 246, (for clarity only one is shown). In a voltage-controlled display voltage rather than current drivers may be employed.
In operation each row of active matrix display 202 is selected in turn using row electrodes 204 and, for each row, the brightness of each pixel in a row is set by the driving column electrodes 208 with brightness data comprising either a current or a voltage. The active matrix pixels including a memory element, generally a capacitor, to keep a row illuminated even when not selected and thus once data has been written to the display, it only needs to be updated with changes to the pixels. Power to the display is provided by a battery 224 and a power supply unit 222 to provide a regulated Vss output 228.
There are many types of OLED pixel driver circuits, for example the improved pixel driver circuits described in UK patent application GB2,381,643. However for the purposes of the following discussion, to illustrate problems common to these driving arrangements, and also to voltage-controlled driver circuits, the simplified circuit model of FIG. 3a is employed.
FIG. 3a shows one pixel 300 of an active matrix color OLED display and its associated drive circuitry. This is reproduced for each pixel of the display. The pixel comprises three sub-pixels 300a, b, c emitting in the red, green, and blue portions of the visible spectrum respectively. Each sub-pixel comprises an OLED 302a, b, c and an associated drive transistor 304a, b, c, typically a thin film transistor; the remainder of the sub-pixel addressing and drive circuitry is not shown. Each of the three sub-pixels and their associated driver transistors are connected between common supply (in this case, Vss) and ground lines. In general these are common for all the pixels of an active matrix display.
FIG. 3b shows a view from above of a display face of an active matrix color display 310 comprising a plurality of pixels 300 and FIG. 3c shows an enlargement of FIG. 3b showing details of a single pixel 300. In this example the red, green, and blue OLEDs 302a, b, c of the sub-pixels 300 a, b, c are formed as three adjacent vertical stripes but it will be appreciated that many other geometrical configurations are possible. Broadly speaking in cross section the display is similar to that shown in FIG. 1 but includes an additional semi-conductor layer or layers immediately adjacent the glass substrate in which the driver circuitry is formed.
For simplicity in FIG. 3c the driver circuitry has been omitted although, in practice, this occupies a portion of the area of each pixel. An aperture ratio may be defined as the active area of the pixel (or sub-pixel) divided by the total available pixel area. Thus in a color display aperture ratios are generally defined such that the sum of the aperture ratio of each sub-pixel equals the total aperture ratio.
Referring again to FIG. 3a, the efficiency of the pixel, that is of the OLED and driver combination, is determined by the intrinsic efficiency of the OLED, usually measured in candelas per amp (cd/A) and by the losses in the OLED drive transistors. These are preferably operated in saturation, and the power supply voltage (in the above example, Vss) is chosen such that when the voltage drop across the driver transistor is taken into account the power supply is just sufficient to drive an OLED at the maximum desired brightness. However in a similar way to a conventional silicon diode once a ‘turn-on’ voltage has been reached only a small additional increase in voltage causes a rapid increase in current through the device so that a drive voltage of an OLED only varies a little with the current through the device and may be considered to be approximately constant. (This is why the drivers in an active matrix pixel such as pixel 300 of FIG. 3a generally provide a controlled current source or sink.)
The configuration of FIG. 3a is convenient but a problem arises when the red, green and blue OLEDs 302a, b, c have very different drive voltages. This can occur when all three OLEDs are fabricated from light emitting polymer, when the blue voltage in particular tends to be significantly higher than the red voltage. However the problem is particularly acute when different classes of material are employed for the different color OLEDs. Thus small-molecule OLEDs tend to require a significantly higher drive voltage than light emitting polymer (LEP)-based devices. In particular, phosphorescent light emitting small molecules tend to have a significantly greater intrinsic efficiency than polymer OLEDs but also tend to require a higher drive voltage. An example of iridium phenylpyridine phosphorescent complexes is described in “Very high-efficiency green organic light-emitting devices based on electrophosphorescence” M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, and S. R. Forrest Applied Physics Letters Vol 75(1) pp. 4-6. Jul. 5, 1999.
A green emitting phosphorescent-based OLED may exhibit a maximum efficiency of, say, 55 cd/A; a blue LEP-based fluorescent OLED may have an efficiency of only around 12 cd/A. These variations in efficiency in part arise from the mode of light emission, that is whether this is primarily fluorescence which relies on singlet excitons, or primarily phosphorescence which additionally utilises triplet excitons leading to increased efficiency.
Light emitting dendrimers comprise a further class of light emitting materials. Light emitting dendrimers comprise a light emitting core surrounded by branched molecular chains termed dendrons. A particularly useful class of phosphorescent light emitting dendrimers is disclosed in WO02/066552.
To create an increased efficiency OLED display it is desirable to be able to combine OLEDs fabricated from different classes of material, such as a dendritic phosphorescent green emitter, a polymer-based blue emitter, and a dendritic phosphorescent or polymer-based red emitter. However although dendritic phosphorescent materials are more efficient than fluorescent systems (both polymers and small molecules), when they are combined with fluorescent materials in a display the overall higher drive voltage required, and hence increased driver losses as described further below, makes the overall display less efficient. For example a green dendritic phosphorescent emitter requires a drive voltage of approximately 7 volts and with, say, four volts dropped across a drive transistor in saturation this dictates a minimum supply voltage of approximately 11 volts. However the drive voltage of blue and red LEP-based devices might only be, say, four volts and three volts respectively. Thus in this example there is a power loss in the blue sub-pixel of three volts times the drive current and in the red sub-pixel four volts times the drive current.
JP2000-089691 describes the series connection of OLEDs to improve power consumption but does not address the problems which arise where light emitting molecules having substantially different drive voltages are used in a display. JP2000-029404 also describes the series connection of organic electroluminescent elements, but to reduce the impact of a short within an element (which would otherwise cause a pixel to go dark) rather than in connection with the above described problem.